[patch, libfortran] AMD-specific versions of library matmul

[Jerry DeLisle]

On 05/25/2017 10:20 AM, Janne Blomqvist wrote:
> On Thu, May 25, 2017 at 1:45 PM, Thomas Koenig <tkoenig@netcologne.de> wrote:
>> Hello world,
>>
>> the attached patch speeds up the library version of matmul for AMD chips
>> by selecting AVX128 instructions and, depending on which instructions
>> are supported, either FMA3 (aka FMA) or FMA4.
>>
>> Jerry tested this on his AMD systems, and found a speedup vs. the
>> current code of around 10%.
>>
>> I have been unable to test this on a Ryzen system (the new compile farm
>> machines won't accept my login yet).  From the benchmarks I have read,
>> this method should also work fairly well on a Ryzen.
>>
>> So, OK for trunk?
> 
> In some comments, you have -mprefer=avx128 whereas the option that gcc
> understands is -mprefer-avx128. Also, have you verified that e.g.
> contemporary Intel processors still use the avx256 codepath and don't
> accidentally end up with avx128?
> 
> As for FMA4, are there sufficient numbers of processors supporting
> FMA4 but not FMA3 around to justify bloating the library to support
> them? I understood that this is only a single AMD CPU generation
> ("bulldozer" in 2011), the next one ("piledriver" in 2012) added FMA3
> in addition to FMA4. And in the new Zen core (Ryzen, Epyc, etc.) AMD
> has dropped support for FMA4 although there are reports that it will
> still execute FMA4 for backward compatibility although it's no longer
> advertised in CPUID, but in any case AMD seems to consider it a legacy
> instruction that should not be used anymore (Intel never supported
> it).
> 

Good questions. I am testing this on Ryzen now. It does work as advertised. The 
cpu flags only advertise FMA.

So I will be testing the older AMD machine which advertises FMA4 and FMA with 
just the FMA flag and likewise the Ryzen with FMA4 and FMA.

I want to see if there is any breakage between the two generations of AMD I can 
access.

Also Ryzen with and without -mprefer-avx128 will be tested.

I do not have an Intel box to test.

Regards,

Jerry